The present invention relates generally to a pump and more particularly to a rotary vane pump, more specifically a pump for steering assistance.
A known rotary vane pump as disclosed for example in German specification No. DE-A-1 528 973 has two displacement regions for displacing the fluid to be pumped, with an inlet opening and an outlet opening, the inlet opening being connected to a hydraulic fluid supply system and the outlet opening being connected to a hydraulic fluid discharge system. The supply and discharge systems communicate with each other by way of a flow control valve for by-passing an excess portion of the displaced flow of the pump into the supply system, the flow control valve including a spool with two spool areas acted upon by respective pressures, a valve spring and an orifice means at which a pressure drop of the controlled output flow of the pump is taken off and passed to the two areas of the spool. The discharge system of the pump has an annular chamber to which there is connected a passage having two passage portions branching therefrom, the directions of flow therein being turned through 90.degree. in each passage. A throat is disposed in the first branch passage orifice and carries the entire displaced flow of the pump. The orifice is disposed in the second branch portion. The design options in regard to the arrangement of the throat and the orifice are limited because of the geometrical factors in the design of the pump. Thus, in that pump, the throat is not arranged coaxially with respect to the spool. The desired output flow is shown to increase slightly in relation to an increasing speed of rotation of the pump.
In another rotary vane pump of the general kind just described above, as disclosed in German specification DE-A-2,001,614, the flow control valve has a first and a second restrictor means in the hydraulic fluid discharge system disposed in succession, for delivering a pressure drop to the flow control valve to permit the achievement of a falling characteristic in respect of the output flow from the pump, in relation to the speed of rotation thereof. With a steering assistance pump of ZF type 7681 produced according to the principles of the just mentioned patent, however, that falling characteristics is only achieved in relation to a pressure range of from 0 to 10 bars. The pump has two displacement regions which respectively communicate with the fluid discharge system by way of outlet openings in the pressure plate of the pump, while disposed upstream of one of the outlet openings is a plug-like throttle insert member as the first restrictor means and which includes an orifice bore as the second restrictor means through which the controlled output flow of the pump is taken off. A disadvantage with that construction is a certain degree of randomness in the flow around the throttle insert member as it is not possible for all the displaced flow to be effective for the restrictor means and therefore for the flow control valve.
In another known rotary vane pump, as disclosed in U.S. Pat. No. 4,199,304, the output flow of the pump to the load connected thereto flows through a restrictor in the form of a venturi throat, the pressure drop of which thus depends on the output flow. The pressure drop produced in that way is passed to the spool by way of a transverse bore and a passage means, so that the spool is moved at a higher level of output flow, to a greater degree than at a lower level of output flow. Accordingly, as the spool is moved to a greater degree, the flow of hydraulic fluid into the inlet of the pump is shut down to a progressively increasing extent and the output flow cannot increase to the degree as would otherwise occur by virtue of the higher speed of rotation of the hydraulic pump. Therefore, the degree to which the output flow increases is reduced, with a given increase in pump speed, whereby the output flow remains relatively constant at higher pump speeds, even if the speed of roation of the pump increases further.
In a situation involving steering assistance, the output flow is returned to the tank by way of the steering valve. When that occurs, the hydraulic fluid which is under pressure, is relieved, which results in a corresponding energy loss if the steering system does not absorb and make use of the power provided thereby. In practical circumstances, such a high level of power utilisation does not occur in the range of high speeds of rotation of the pump, because it is not possible to make sharp steering motions when travelling quickly. Accordingly, in the high range of pump rotation, the system maintains a condition of constant power output readiness which is not required at that level and which thus results in an unnecessary energy loss.